The present invention relates generally to a system for controlling intersection traffic signals in response to the presence of an approaching emergency vehicle. More particularly, the present invention relates to a system for providing a pre-emption signal output to an intersection traffic controller which pre-empts the existing status of the traffic lights in response to the approach of an emergency vehicle to an intersection under an emergency condition.
Under modern day traffic conditions, and particularly in high density traffic areas, there is a need for a system to modify the normal operating sequence of intersection traffic signals to permit the unimpeded and safe passage of emergency vehicles through the intersection. Not only must the emergency vehicle be free to move through the intersection in a selected direction, but the traffic flow from other, potentially interfering traffic directions, must be stopped under the appropriate traffic signal. A number of traffic control systems have been developed to accomplish this purpose, but each has, for one reason or another, fallen short of the desired result. A common approach has been to provide a transmitter associated with the emergency vehicle for transmitting an emergency signal to a receiver associated with the traffic signal controller whereby the traffic signal controller is modified by the transmitted signal to operate the traffic signals in an emergency sequence. As an example of the foregoing, see, e.g., U.S. Pat. No. 4,228,419 ("Obeck"), U.S. Pat. No. 4,017,825 ("Pinchey"), U.S. Pat. No. 4,223,295 ("Bonner"), and U.S. Pat. No. 4,573,049 ("Anderson"). Yet another type of traffic signal preempt system, described in U.S. Pat. No. Re 28,100 ("Long"), discloses a system using a light communication signal transmitted from the emergency vehicle which causes the traffic light signal controller to receive the light communication signal and control the traffic signal operation in a selected phase in response to a detected approach direction. Still another modification of a vehicle based transmission system is disclosed in U.S. Pat. No. 4,296,400 in which the vehicle is equipped with devices for automatically determining the position of the vehicle and includes a transmission receiver antenna which transmits in a direction perpendicular to the travel direction and transmits bearing information, while a second antenna is arranged to unidirectionally transmit in the direction of travel and transmits transfer commands to control the traffic light signals.
Regardless of the emergency signal transmitting system utilized, the conventional practice in all of these systems is to use a vehicle-based transmitter which activates an intersection-based receiver, linked to the traffic signal control circuitry, to control the sequence of the traffic signal at the intersection. Of the foregoing described systems, the Bonner and Anderson patents disclose emergency traffic signal control systems employing microwave signals, in which at least one directional microwave antenna is mounted at an intersection; the emergency vehicle transmits an encoded microwave signal, which is received by the intersection antenna, decoded and fed to a preempt system associated with the traffic signal controller. The Obeck patent adds a transponder to the intersection receiver which confirms receipt of the transmitted signal to the vehicle.
Those skilled in the art will understand and appreciate that each of the foregoing conventional emergency vehicle traffic signal control systems requires the emergency vehicle to provide the first affirmative signal to which the intersection controller is responsive.